This invention relates to fabrication of film resistors and the resulting product, and in particular to a method which results in stable resistance characteristics, an ability of film resistors to withstand high current surges or voltage spikes, and precise trimming of resistors of small physical size.
Thin and thick film resistor circuits are now used in a wide variety of applications. Full use of such resistors has, however, been sometimes limited by the fact that the resistance of such elements usually has to be adjusted to a desired value by a cutting operation. Laser trimming is now a standard technique in the industry and gives adequate results for most applications. However, problems are created and explained below.
FIG. 1 illustrates a typical rectangular geometry resistor (10), with electrodes (11 and 12) making contact thereto, formed on an insulating substrate 16. The resistor has been laser-trimmed by standard techniques. The area of the trim cut is shown as 13. Current flow is illustrated as dashed lines 14. It will be noted that since the current is constrained to a narrow portion of the resistor, the current density in the vicinity of the cut, illustrated as area 15, increased. This current crowding has heretofore precluded use of film resistors which are exposed to large current surges, such as current limiting resistors exposed to lightning surges in the telephone loop plant. This effect has also caused problems where the film resistors are used as part of voltage divider networks in relay circuits. High voltages applied to the resistors in these circuits result in unacceptable resistance changes. Furthermore, even in the absence of current or voltage surges, again processes occur in the vicinity of the cut contributing to a change in resistance.
In addition to the problem of withstanding high currents or voltages, the prior art methods also limit the size of resistors which can be precisely trimmed. If too narrow a current path exists at the end of the trim cut, the resistance will drift excessively due to aging effects in the vicinity of the trim cut.
It is therefore a primary object of the invention to provide a trimming technique which permits a film resistor to withstand high currents and voltages. It is a further object of the invention to provide a technique which allows fabrication of small, precisely trimmed resistors. It is a still further object of the invention to produce resistors in general with highly stable resistances.